1. Field of the Invention
The present invention relates to a resist coating film for use at the time of forming a fine pattern of a semiconductor device, and to a pattern forming method and a semiconductor device.
2. Description of the Related Art
In the recent tendency of desiring highly integrated semiconductor devices, micro-processing technology has been improved. In order to realize the foregoing technology, some means have been investigated. Among the foregoing means, exposure technology has attracted attention, the technology using an excimer laser as the exposure light source to shorten the wavelength of the light source for use in the photolithography technology. Another exposure technology using, as the exposure light source, electron beams or X-rays, with which problems of optical interference can be ignored, has attracted attention.
With the tendency of shortening the wavelength of the exposure light source, there arise problems of nonconformance of photosensitive wavelength with respect to a resist and unsatisfactory transmittance of the resist with respect to the short wavelength. As a result, improvement in resolution has been difficult. In order to overcome the foregoing problems, use of a resist utilizing a chemical amplification mechanism has been investigated. As disclosed in Japanese Patent Laid-Open No. 59-45439, a high resolution pattern can be obtained by using a resist composition containing: a polymer of a type in which a water-soluble functional group of a resin exhibiting high transparency with respect to far ultraviolet rays is substituted by a group which is unstable with respect to acids; and a compound that generates acids due to irradiation with ultraviolet rays.
The lithography technology using far ultraviolet rays, such as excimer laser beams, employs a three-component chemical amplification positive-type resist composed of: an acid generating agent which is decomposed due to a photochemical reaction to generate acids; a base resin having a group which does not considerably absorb far ultraviolet rays and which is decomposed due to an acid-catalyst reaction, to improve the solubility thereof; and a dissolution inhibitor or a two-component chemical amplification positive-type resist that does not contain the dissolution inhibitor.
FIGS. 5A to 5D illustrate an example of the chemical amplification positive-type resist. FIG. 5A illustrates a structural formula of the base resin, FIG. 5B illustrates that of the dissolution inhibitor for inhibiting the solution of the base resin in an alkali developer, and FIG. 5C illustrates that of the acid generator.
As an example of poly-p-hydroxystyrene derivative for use as the base resin, poly-p-hydroxystyrene that has been made partially esterified with a tertiary butoxy carbonyl group (hereinafter called "t-Boc") is exemplified. Referring to the drawing, n is a natural number denoting the degree of polymerization, x is a number which is zero or more and as well as less than 1 and which denotes the ratio made to be t-Boc. Specifically n is a value from 0.1 to 0.5 (see FIG. 5A). As an example of the dissolution inhibitor, the general formula of bis phenol A-type is shown. Referring to the drawing, R is an alkyl group (see FIG. 5B). In FIG. 5C, which illustrates an example of the acid generating agent, .sup.- B is the paired anion of a triallylsulfonium cation, the structure of which is shown in FIG. 5D. Symbol M is a metal element such as arsenic or antimony, X is a halogen element, and subscript n is the acid digit of the metal element.
The method of forming a conventional resist pattern will now be described with reference to the drawings. FIGS. 6A to 6D are cross sectional views which illustrate the conventional method of forming a resist pattern by using a chemical amplification positive-type resist and utilizing excimer laser beam irradiation, the method being illustrated in the sequential order of the process.
First, a chemical amplification positive-type resist film 2 is, after spin coating has been performed, formed on a semiconductor substrate 1 to have a thickness of about 1.0 to 1.5 .mu.m by soft baking performed at about 80.degree. C. to 130.degree. C. (see FIG. 6A).
Then, the resist film 2 is selectively irradiated with excimer laser beams 3 from a position above the semiconductor substrate 1 through a reticle 4 so that proton acid 5 is generated, the proton acid 5 catalyzing a de-t-Boc reaction of the base resin, which has been made partial t-Boc, and the dissolution inhibitor (see FIG. 6B).
Thereafter, the semiconductor substrate 1 is heated by baking at about 60.degree. C. to 100.degree. C. for 1 to 2 minutes so that a portion 6 of the resist film 2 irradiated with excimer laser beams 3 is caused to de-t-Boc-react selectively the base resin and the dissolution inhibitor due to the acid catalyst. As a result, the solubility with respect to the alkali developer is improved (see FIG. 6C).
Then, the portion 6 of the resist film 2 irradiated with excimer laser beams 3 is eluted by a developing fluid such as water-soluble alkali solution having a proper concentration so that a resist pattern 7 is formed (see FIG. 6D).
FIGS. 7A and 7B illustrate the de-t-Boc reactions of the base resin and the dissolution inhibitor due to the acid catalyst taking place in the portion 6 of the resist film 2 irradiated with excimer laser beams 3 in the process shown in FIG. 6C. FIGS. 7A and 7B, respectively, illustrate the de-t-Boc reactions of the dissolution inhibitor and the base resin caused by acid HX generated due to the irradiation of the excimer laser beams 3.
FIG. 8 shows the improvement in the solubility of the exposed portion of the chemical amplification positive-type resist with respect to the alkali developer due to the foregoing de-t-Boc reaction. FIG. 8 is a graph which shows the difference in the solubility between a non-exposed portion of the chemical amplification positive-type resist and the exposed portion of the same with respect to the alkali developer.
Since the formation of the fine pattern by using the conventional chemical-amplification-type resist is performed as described above, high sensitivity and high resolution can be realized. However, the acid or its precursor generated due to the exposure reacts with oxygen in air and basic compounds present in the atmosphere and loses its activity in the surface of the resist film that is in contact with the atmosphere. In particular, the foregoing tendency becomes excessive if a long time passes from the exposure process to the ensuing heating process.
FIGS. 9A to 9C are graphs which illustrate the state in the surface portion of the resist film. FIG. 9A illustrates the change in the concentration of the acid, FIG. 9B illustrates the change in the solubility with respect to the alkali developer and FIG. 9C is a cross sectional view of the resist pattern.
As shown in FIG. 9A, the concentration of the acid falls in a portion from the surface of the resist film designated by symbols .DELTA.x. As a result, a reaction of decomposing a non-polar group or a reaction of a cross linking agent does not take place effectively in the heating process to be performed after the exposure has been performed. Hence, the sensitivity excessively deteriorates and the solubility with respect to the alkali developer is excessively lowered in the surface portion of the resist film as shown in FIG. 9B. Therefore, a layer which cannot be dissolved easily and which has the thickness .DELTA.x is formed in the surface of the resist film. As a result, there arises a problem that the resist pattern 7, which is formed into an accurate rectangle and the dimensions of which can be controlled satisfactorily, cannot be formed. Therefore, although the chemical-amplification-type resists are employed only to manufacture prototypes of semiconductor devices or to perform experiments, it cannot be employed in mass production.